U.S. patent number 8,220,066 [Application Number 12/672,022] was granted by the patent office on 2012-07-10 for vibration compensation in probe microscopy.
This patent grant is currently assigned to Infinitesima Ltd.. Invention is credited to Andrew Humphris.
United States Patent |
8,220,066 |
Humphris |
July 10, 2012 |
Vibration compensation in probe microscopy
Abstract
A The local probe microscopy apparatus (1) comprises a probe (3)
with translation stages (5a, 5b) for controlling the position of
the probe (3) relative to a sample surface. The probe (3) has a
feedback mechanism (6, 5 7) for maintaining the deflection of the
probe and a height measuring system (9) which includes means for
compensating for environmental noise. The local probe microscopy
apparatus is particularly suitable for use as a wafer inspection
tool in a wafer fabrication plant where the inspection tool is
liable to be exposed to significant mechanical vibration.
Inventors: |
Humphris; Andrew (Oxfordshire,
GB) |
Assignee: |
Infinitesima Ltd. (Oxfordshire,
GB)
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Family
ID: |
38529211 |
Appl.
No.: |
12/672,022 |
Filed: |
August 4, 2008 |
PCT
Filed: |
August 04, 2008 |
PCT No.: |
PCT/GB2008/050660 |
371(c)(1),(2),(4) Date: |
March 31, 2010 |
PCT
Pub. No.: |
WO2009/019513 |
PCT
Pub. Date: |
February 12, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20100235955 A1 |
Sep 16, 2010 |
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Foreign Application Priority Data
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Aug 3, 2007 [GB] |
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0715102.0 |
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Current U.S.
Class: |
850/6; 356/487;
850/55; 850/1; 850/19; 356/493; 850/26; 850/18; 850/33; 850/4 |
Current CPC
Class: |
G01Q
20/02 (20130101); G01Q 70/04 (20130101) |
Current International
Class: |
G01Q
20/02 (20100101) |
Field of
Search: |
;850/1,6,4,18,19,26,33,55 ;356/487,493 ;250/235 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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4344499 |
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Jul 1995 |
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DE |
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0422548 |
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Apr 1991 |
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EP |
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59168304 |
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Sep 1984 |
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JP |
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05079834 |
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Mar 1993 |
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JP |
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05187866 |
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Jul 1993 |
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JP |
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06082250 |
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Mar 1994 |
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JP |
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Other References
"Surface Profilometer with Ultra-High Resolution," IBM Technical
Disclosure Bulletin, vol. 35, No. 3, Aug. 1, 1992, pp. 207-208,
XP000326239. cited by other.
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Primary Examiner: Berman; Jack
Assistant Examiner: Sahu; Meenakshi
Attorney, Agent or Firm: Volpe and Koenig, P.C.
Claims
The invention claimed is:
1. A method of compensating for vibration in a local probe
measurement apparatus, wherein the method comprises the steps of
scanning a scanning probe relative to the surface of the sample;
reflecting a probe beam off a reflective point having a fixed
spatial relationship with respect to the scanning probe;
simultaneously reflecting a reference beam off a fixed reference
point subject to vibration substantially the same as the vibration
experienced by the sample; and interferometrically combining the
reflected probe beam and the reflected reference beam to generate
differential probe height data which incorporates compensation for
the vibration, the differential probe height data being
representative of either changes in the interaction between the
scanning probe and the sample surface or the topography of the
sample surface.
2. A method as claimed in claim 1, wherein the reference beam is
reflected from a predetermined fixed point on one of: the sample,
the sample stage and the sample stage support.
3. A method as claimed in claim 1, wherein the measurement
apparatus includes a reference probe and the reference beam is
reflected from a predetermined fixed point having a fixed spatial
relationship relative to the reference probe.
4. A method as claimed in claim 1, wherein the method further
comprises monitoring the interaction between the scanning probe and
the sample surface and adjusting the position of the scanning probe
to maintain a substantially constant interaction.
5. A method as claimed in claim 4, wherein the deflection of the
scanning probe is monitored and the position of the scanning probe
is adjusted to maintain a substantially constant deflection.
6. A method as claimed in claim 1, further comprising monitoring
the separation of a first fixed predetermined position from a
second predetermined position on one of the sample, sample stage
and sample stage support wherein the separation is substantially
parallel to one or other of the scan axes.
7. A method as claimed in claim 6, wherein a fixed sensor is
provided to monitor the separation.
8. A method as claimed in claim 7, wherein the fixed sensor is an
optical displacement detector.
9. A method as claimed in claim 6, wherein separations between at
least two pairs of predetermined fixed positions are monitored, the
separation between the first pair of fixed positions being
substantially orthogonal to the separation between the second pair
of fixed positions.
10. A method as claimed in claim 7, wherein the fixed sensor
comprises an interferometric device employing a reference beam
reflected off a reference point having a fixed spatial relationship
with respect to the probe of the microscopy apparatus, the detected
changes in the separation comprising differential lateral
separation data.
11. A method as claimed in claim 1, further comprising the step of
using a separate scanning reference probe to scan a predefined
reference structure which is subject to vibration substantially the
same as the vibration experienced by the sample, the shape of the
reference structure being defined to enable detection by the
scanning reference probe of changes in the position of the
reference structure parallel to the scanning plane.
12. A method as claimed in claim 6, wherein the trajectory of the
scanning probe is adjusted in response to detected changes in the
one or more monitored separations or the position of the reference
structure.
13. A method as claimed in claim 6, wherein the differential probe
height data is recorded as data points in a pixellated array and
the data points within the pixellated array are adjusted in
dependence upon detected changes in the one or more monitored
separations or the position of the reference structure.
14. A method of correcting for vibration in a local probe
measurement apparatus, wherein the method comprises detecting
changes in the separation of a first fixed predetermined position
from a second predetermined position on one of the sample, sample
stage and sample stage support representative of vibration wherein
the separation is substantially parallel to one or other of the
scan axes.
15. A method as claimed in claim 14, further comprising,
simultaneously with the step of detecting changes in the
separation, scanning a scanning probe relative to the surface of
the sample in an imaging plane of the sample and monitoring changes
in height of the scanning probe either representative of changes in
the interaction between the scanning probe and the sample surface
or representative of the topography of the sample surface.
16. A method as claimed in claim 14, wherein a fixed sensor is
provided to monitor the separation.
17. A method as claimed in claim 16, wherein the fixed sensor is an
optical displacement detector.
18. A method as claimed in claim 14, wherein separations between at
least two pairs of predetermined fixed positions are monitored, the
separation between the first pair of fixed positions being
substantially orthogonal to the separation between the second pair
of fixed positions.
19. A method as claimed in claim 16, wherein the fixed sensor
comprises an interferometric device employing a reference beam
reflected off a reference point having a fixed spatial relationship
with respect to the probe of the microscopy apparatus, the detected
changes in the separation comprising differential lateral
separation data.
20. A method as claimed in claim 15, wherein the trajectory of the
scanning probe is adjusted in response to detected changes in the
one or more monitored separations.
21. A method as claimed in claim 15, wherein changes in the height
of the scanning probe are recorded as data points in a pixellated
array and the data points within the pixellated array are adjusted
in dependence upon detected changes in the one or more monitored
separations.
22. A local probe microscopy apparatus comprising a scanning probe;
scanning means for scanning the probe relative to the surface of a
sample which is subject to externally applied vibration; an imaging
device for reflecting a probe beam off a reflective point having a
fixed spatial relationship with respect to the scanning probe and
for simultaneously reflecting a reference beam off a fixed
reference point subject to vibration substantially the same as the
vibration experienced by the sample; and an interferometric device
for interferometrically combining the reflected probe beam and the
reflected reference beam to generate differential probe height data
which incorporates compensation for vibration experienced by the
sample, the differential probe height data being representative of
either changes in the interaction between the scanning probe and
the sample surface or the topography of the sample surface.
23. The local probe microscopy apparatus as claimed in claim 22,
further comprising a probe interaction detector for monitoring the
interaction between the scanning probe and the sample surface and
for adjusting the position of the scanning probe to maintain a
substantially constant interaction.
24. The local probe microscopy apparatus as claimed in claim 23,
wherein the probe interaction detector is a probe deflection
detector for monitoring the deflection of the scanning probe and a
feedback mechanism in communication with the probe deflection
detector for maintaining a substantially constant deflection of the
scanning probe.
25. The local probe microscopy apparatus as claimed in claim 22,
further comprising a fixed reference probe wherein the reference
beam is reflected off a predetermined reference point having a
fixed spatial relationship with respect to the reference probe.
26. The local probe microscopy apparatus as claimed in claim 22,
further comprising a position monitoring device adapted to detect
changes in the separation of a first fixed predetermined position
from a second predetermined position on one of the sample, sample
stage and sample stage support representative of vibration wherein
the separation is substantially parallel to one or other of the
scan axes.
27. The local probe microscopy apparatus as claimed in claim 26,
wherein the position monitoring device includes a fixed sensor
located to monitor changes in the separation between the first and
second predetermined fixed positions.
28. The local probe microscopy apparatus as claimed in claim 27,
wherein the fixed sensor is an optical displacement sensor arranged
such that at least part of a reference beam extending between the
optical displacement sensor and one of the predetermined fixed
positions is substantially parallel to one of the scan axes of the
probe.
29. The local probe microscopy apparatus as claimed in claim 26,
wherein the position monitoring device is adapted to detect changes
in the separations between at least two pairs of predetermined
fixed positions, the separation between the first pair of fixed
positions being substantially orthogonal to the separation between
the second pair of fixed positions.
30. The local probe microscopy apparatus as claimed in claim 27,
wherein the fixed sensor comprises an interferometric device
employing a reference beam reflected off a reference point having a
fixed spatial relationship with respect to the probe of the
microscopy apparatus, the detected changes in the separation
comprising differential lateral separation data.
31. A local probe microscopy apparatus comprising a position
monitoring assembly adapted to detect changes in the separation of
a first fixed predetermined position from a second predetermined
position on one of the sample, sample stage and sample stage
support representative of vibration wherein the separation is
substantially parallel to one or other of the scan axes.
32. The local probe microscopy apparatus as claimed in claim 31,
wherein the position monitoring assembly includes at least two
fixed sensors adapted to detect changes in the respective
separations between at least two pairs of predetermined fixed
positions, the separation between the first pair of fixed positions
being substantially orthogonal to the separation between the second
pair of fixed positions.
33. The local probe microscopy apparatus as claimed in claim 31,
wherein the at least one fixed sensor is an optical displacement
sensor.
34. The local probe microscopy apparatus as claimed in claim 31,
wherein the at least one fixed sensor comprises an interferometric
device employing a reference beam reflected off a reference point
having a fixed spatial relationship with respect to the probe of
the microscopy apparatus, the detected changes in the separation
comprising differential lateral separation data.
Description
FIELD OF THE INVENTION
The present invention generally relates to vibration compensation
in probe microscopy and to a method of and apparatus for vibration
compensation. The present invention is particularly, but not
exclusively, suited for use in the inspection and/or review of
semiconductor wafers and to a surface review tool for use in
surveying and/or imaging specimen surfaces using a local probe. A
particular type of local probe microscope and a field of
application for which the invention is suitable is scanning force
microscopy (SFM), also know as atomic force microscopy (AFM).
However, it is to be understood that the invention is not
restricted to such a field.
BACKGROUND AND RELATED ART
Local probe microscopy is capable of high-resolution
non-destructive measurement of one or more characteristics of a
specimen's surface. A typical AFM operated in contact mode includes
a probe comprising a cantilever with a probe tip mounted near the
free end of the cantilever. The AFM probe is mounted on a stage
having actuators which provide x, y and z motion of the probe tip
relative to a sample stage. An optical lever system (laser
reflection from the back of the cantilever onto a segmented
photodiode) or an alternative position sensitive detector is used
to monitor deflection of the free end of the cantilever which
arises from changes in the interaction between the probe tip and
the surface of a sample on the sample stage. In constant force
scanning, as the probe is moved in the x, y plane so as to scan
across the surface of the sample, the height of the probe above the
sample, i.e. in the z direction, is adjusted to maintain a fixed
deflection of the cantilever. These z direction adjustments
maintain a constant interaction force between the probe tip and the
sample surface for a particular x, y location and are
representative, for example, of the topography of the sample
surface. With a conventional AFM of the type described above
lateral and vertical resolutions of better than 2 nm and 0.1 nm
respectively are achievable. Under the correct conditions, lateral
resolution down to the atomic level can be achieved. However, the
operation and performance of the microscope is dependent upon a
knowledge of the spatial relationship between the sample and the
probe. Inevitably this spatial relationship is affected, and thus
the resolution is limited, by amongst other things the operating
environment and in particular mechanical and acoustic noise which
generate an uncertainty in the position of the probe relative to
the specimen.
Various improvements have been proposed in recent years to address
the problem of noise with respect to local probe microscopy. In
U.S. Pat. No. 6,196,061 and U.S. Pat. No. 6,279,389, for example,
an atomic force microscope (AFM) is described which employs two
separate probes: one a measurement probe, the other a reference
probe mounted on opposite corners of a single cantilever. The
reference probe is flattened (made dull) so as to average the
specimen topography as the flattened probe is scanned over the
specimen. Additionally, the microscope includes a structure which
mechanically links the specimen platform to the structure
supporting the local probe. Deflection of the cantilever is
measured optically from a point on the back of the cantilever
midway between the two probes and thus generates a signal with a
reduced noise component that provides differential deflection
measurements with respect to the surface area adjacent the
measurement probe and thus is not independent of the scanning
measurements. However, the AFM is restricted to differential
deflection data; no absolute deflection measurements of the
measurement probe can be produced using this microscope. These two
patents also describe an alternative probe consisting of multiple
cantilevers arranged parallel to one another with interdigitated
fingers along each side edge. The interdigitated fingers act as
diffraction gratings again providing differential deflection
data.
In US 2003/0020500 and US 2005/0040836 a probe having two or more
probe tips each individually mounted on a respective cantilever but
with common position control is described. The individual probe
tips are arranged so that one tip, a distance sensor, is positioned
lower, i.e. closer to the sample stage, than the remaining probe
tips. Unlike U.S. Pat. No. 6,196,061 and U.S. Pat. No. 6,279,389,
with this version of a probe having multiple cantilevers, the
deflection of each cantilever is separately monitored using, for
example, a conventional optical lever technique. The distance
sensor provides a reference height measurement from the sample
surface and is used to maintain the distance of the remaining probe
tips in the z direction from the surface of the sample. This
permits the surface of the sample to be scanned using the probe
tips at well defined distances above the surface of the sample.
However, as a result of the common position control, the height
reference measurement of the distance sensor is not independent of
the scanning measurements.
Semiconductor device fabrication involves highly complex techniques
for constructing integrated circuits using semiconductor materials.
Due to the large scale of circuit integration and the decreasing
size of semiconductor devices, the semiconductor fabrication
process is prone to processing defects. Current testing procedures,
involving separate inspection and review processes, are therefore
critical to maintain quality control. Since the testing procedures
are an integral and significant part of the fabrication process,
the semiconductor industry is constantly seeking for more accurate
and efficient monitoring procedures.
The inspection process involves an initial scan of the surface of
the semiconductor wafer by a high-speed, relatively low-resolution
inspection tool. A defect map is then produced showing suspected
locations on the wafer having a high probability of a defect. When
a review of affected wafers is warranted, either the optics of the
inspection apparatus must be changed to a higher (review)
resolution or the wafers are transferred to a different tool
altogether. To perform the review, the defect map is fed to the
review apparatus and then individual suspected defect sites are
more closely scrutinised by means of a relatively high-resolution
image. That image is analysed, for example involving the use of
pattern recognition techniques in the case of automated defect
classification (ADC), to determine the nature of the defect i.e. to
classify the defect in order to discriminate between, for example,
a defective pattern, damage caused by a foreign particle or a
surface scratch.
Optical methods such as dark and bright field microscopy have been
used in wafer defect review and classification processes. However,
such optical devices are limited in their ability to analyze and
accurately identify defect types on a semiconductor wafer. This is
especially the case as the thrust of future development in the
semiconductor device field is to ever higher densities of
integrated circuits, involving sub-micron and sub 100 nm device
features, as well as structures with increased aspect ratios. As a
result, due to its greater resolution, scanning electron microscopy
or "SEM" is favoured in defect review and classification of
features on the wafer. However, scanning electron microscopy has
the disadvantage that not only must the wafers be removed from the
production line; they have to be transferred to an evacuated
enclosure for the purposes of electron microscopy. This can add
considerable time to the inspection and review procedure, which
increased time is undesirable. Moreover, scanning electron
microscopy can result in damage to the wafer surface via beam
damage and/or contamination.
Local probe microscopy techniques, such as those described above,
are capable of providing high resolution information on surface
features of a sample. However, due to the length of time required
to produce a surface scan using conventional scanning probe
microscopy apparatus, the application of such microscopy techniques
is limited in large-scale wafer fabrication. Moreover, as with all
local probe techniques, the resolution and capabilities of such
local probe microscopy apparatus are directly affected by
environmental mechanical vibrations, especially where a local probe
is mounted in a conventional inspection/review tool. The effect of
environmental mechanical vibrations is increased when using a long
mechanical path between the probe and sample as is required for
large samples such as wafers used in semiconductor device
fabrication. Additionally, the need to move and reposition the
sample, e.g. a wafer, introduces mechanical vibrations which must
settle before a measurement can be collected and this reduces
throughput.
Scanning probe microscopes which employ interferometers for
vibration and noise cancellation have been described in U.S. Pat.
No. 6,178,813 and U.S. Pat. No. 7,249,002. In each of these
documents in addition to the usual scanning probe system an
interferometer is used to monitor the separation between a point on
the sample surface and a fixed point. Measured variations in this
separation are used to compensate for noise in the measurements
produced by the scanning probe. However, microscopes employing
these noise compensation techniques require two separate
measurement systems and are thus costly. More importantly the
interferometers are only capable of measuring displacements,
representative of noise amplitudes, of less than half a
wavelength.
SUMMARY OF INVENTION
The present invention is specifically concerned with providing
vibration compensation for use with local probe microscopy
apparatus which is suitable, but not exclusively so, for use in a
semiconductor device fabrication plant.
The present invention therefore provides a method of compensating
for vibration in a local probe measurement apparatus which would
otherwise create uncertainty in the position between the probe and
a sample, wherein the method comprises the steps of scanning a
scanning probe relative to the surface of the sample; reflecting a
probe beam off a reflective point having a known spatial
relationship with respect to the scanning probe; simultaneously
reflecting a reference beam off a fixed reference point subject to
vibration substantially the same as the vibration experienced by
the sample; and interferometrically combining the reflected probe
beam and the reflected reference beam to generate differential
probe height data which incorporates compensation for the
vibration, the differential probe height data being representative
of either changes in the interaction between the scanning probe and
the sample surface or the topography of the sample surface.
Preferably the reference beam is reflected from a predetermined
fixed point on one of: the sample, the sample stage and the sample
stage Support. Alternatively the measurement apparatus may include
a reference probe and the reference beam is reflected from a
predetermined fixed point having a known spatial relationship
relative to the reference probe.
In one embodiment the method further comprises monitoring the
interaction between the scanning probe and the sample surface and
adjusting the position of the scanning probe to maintain a
substantially constant interaction. Ideally the deflection of the
scanning probe is monitored and the position of the scanning probe
is adjusted to maintain a substantially constant deflection.
In a further preferred embodiment the method further comprises
monitoring the separation of a first fixed predetermined position
from a second predetermined position on one of the sample, sample
stage and sample stage support wherein the separation is
substantially parallel to one or other of the scan axes.
A fixed sensor may be provided to monitor the separation and the
fixed sensor may be an optical displacement detector.
Alternatively, the fixed sensor may comprise an interferometric
device employing a reference beam reflected off a reference point
having a predictable spatial relationship with respect to the probe
of the microscopy apparatus, the detected changes in the separation
comprising differential lateral separation data. Also, separations
between at least two pairs of predetermined fixed positions may be
monitored, the separation between the first pair of fixed positions
being substantially orthogonal to the separation between the second
pair of fixed positions.
In a further alternative embodiment a separate scanning reference
probe is used to scan a predefined reference structure which is
subject to vibration substantially the same as the vibration
experienced by the sample, the shape of the reference structure
being defined to enable detection by the scanning reference probe
of changes in the position of the reference structure parallel to
the scanning plane.
With these embodiments the trajectory of the scanning probe may be
adjusted in response to detected changes in the one or more
monitored separations or the position of the reference structure.
Alternatively, the differential probe height data may be recorded
as data points in a pixellated array and the data points within the
pixellated array are adjusted, for example in terms of their
placement, in dependence upon detected changes in the one or more
monitored separations or the position of the reference
structure.
In a separate aspect the present invention provides a method of
correcting for vibration in a local probe measurement apparatus,
wherein the method comprises detecting changes in the separation of
a first fixed predetermined position from a second predetermined
position on one of the sample, sample stage and sample stage
support representative of vibration wherein the separation is
substantially parallel to one or other of the scan axes.
Ideally the method further comprises, simultaneously with the step
of detecting changes in the separation, scanning a scanning probe
relative to the surface of the sample in an imaging plane of the
sample and monitoring changes in height of the scanning probe
either representative of changes in the interaction between the
scanning probe and the sample surface or representative of the
topography of the sample surface.
A fixed sensor may be provided to monitor the separation which may
be an optical displacement detector or may be an interferometric
device employing a reference beam reflected off a reference point
having a fixed spatial relationship with respect to the probe of
the microscopy apparatus, the detected changes in the separation
comprising differential lateral separation data.
Optionally, separations between at least two pairs of predetermined
fixed positions are monitored, the separation between the first
pair of fixed positions being substantially orthogonal to the
separation between the second pair of fixed positions.
In either case the trajectory of the scanning probe may be adjusted
in response to detected changes in the one or more monitored
separations or changes in the height of the scanning probe may be
recorded as data points in a pixellated array and the placement of
the data points within the pixellated array is adjusted in
dependence upon detected changes in the one or more monitored
separations.
In a third aspect the present invention provides local probe
microscopy apparatus comprising a scanning probe; scanning means
for scanning the probe relative to the surface of a sample which is
subject to externally applied vibration; an imaging device for
reflecting a probe beam off a reflective point having a fixed
spatial relationship with respect to the scanning probe and for
simultaneously reflecting a reference beam off a fixed reference
point subject to vibration substantially the same as the vibration
experienced by the sample; and an interferometric device for
interferometrically combining the reflected probe beam and the
reflected reference beam to generate differential probe height data
which incorporates compensation for vibration, the differential
probe height data being representative of either changes in the
interaction between the scanning probe and the sample surface or
the topography of the sample surface.
Preferably, the local probe microscopy apparatus further comprises
a probe interaction detector for monitoring the interaction between
the scanning probe and the sample surface and for adjusting the
position of the scanning probe to maintain a substantially constant
interaction. Ideally, the probe interaction detector is a probe
deflection detector for monitoring the deflection of the scanning
probe and a feedback mechanism in communication with the probe
deflection detector for maintaining a substantially constant
deflection of the scanning probe.
Optionally, the apparatus further comprises a fixed reference probe
wherein the reference beam is reflected off a predetermined
reference point having a fixed spatial relationship with respect to
the reference probe. In a preferred embodiment the apparatus
further comprises a position monitoring device adapted to detect
changes in the separation of a first fixed predetermined position
from a second predetermined position on one of the sample, sample
stage and sample stage support representative of vibration wherein
the separation is substantially parallel to one or other of the
scan axes. The position monitoring device may include a fixed
sensor located to monitor changes in the separation between the
first and second predetermined fixed positions. The fixed sensor
may be an optical displacement sensor arranged such that at least
part of a reference beam extending between the optical displacement
sensor and one of the predetermined fixed positions is
substantially parallel to one of the scan axes of the probe.
Alternatively the fixed sensor may be an interferometric device
employing a reference beam reflected off a reference point having a
fixed spatial relationship with respect to the probe of the
microscopy apparatus, the detected changes in the separation
comprising differential lateral separation data.
In a further preferred embodiment the position monitoring device is
adapted to detect changes in the separations between at least two
pairs of predetermined fixed positions, the separation between the
first pair of fixed positions being substantially orthogonal to the
separation between the second pair of fixed positions.
In a fourth aspect the present invention provides a local probe
microscopy apparatus comprising a position monitoring assembly
adapted to detect changes in the separation of a first fixed
predetermined position from a second predetermined position on one
of the sample, sample stage and sample stage support representative
of vibration wherein the separation is substantially parallel to
one or other of the scan axes.
In a preferred embodiment the position monitoring assembly includes
at least two fixed sensors adapted to detect changes in the
respective separations between at least two pairs of predetermined
fixed positions, the separation between the first pair of fixed
positions being substantially orthogonal to the separation between
the second pair of fixed positions.
The at least one fixed sensor may be an optical displacement sensor
or may be an interferometric device employing a reference beam
reflected off a reference point having a fixed spatial relationship
with respect to the probe of the microscopy apparatus, the detected
changes in the separation comprising differential lateral
separation data.
It is to be understood that reference herein to a scanning probe
and to a probe which scans a specimen or sample is intended to
encompass any probe which is adapted to collect measurements or
data representative of characteristics of the specimen or sample at
a plurality of different positions on the surface of the specimen
or sample i.e. different x and/or y and/or z positions. Hence,
reference herein to a scan line is intended to encompass a linear
path over the surface of a specimen or sample along which a probe
tip gathers information concerning the specimen or sample. The
information gathered along the scan line is used to produce an
image of the characteristics of the specimen or sample, the image
having a conformation and orientation corresponding to the
conformation and orientation of the scan line and which, in the
case of digital images, will be formed of a plurality of
pixels.
The present invention is applicable to the whole range of local
probe techniques, including contact mode and intermittent-contact
and non-contact dynamic modes of operation for the AFM and other
scanning probe microscopy (SPM) techniques which are all sensitive
to vibration. Since it is not possible to give a complete list of
SPM techniques to which the invention can be applied, only some
important techniques are listed below as non-limiting examples:
scanning tunnelling microscopy (STM), magnetic force microscopy
(MFM), scanning capacitance microscopy (SCM), scanning near-field
microscopy (SNOM), lateral force microscopy (LFM) and electric
field/force microscopy (EFM).
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described by way of example only
with reference to the accompanying drawings, in which:
FIG. 1 is a schematic overview of one embodiment of scanning probe
microscopy apparatus in accordance with the present invention;
FIG. 2 is a schematic overview of a wafer review tool employing a
scanning local probe technique and noise compensation in accordance
with the present invention;
FIG. 3 is a schematic view of a noise compensation design in
accordance with the present invention;
FIG. 4 is a schematic view of an alternative noise compensation
design in accordance with the present invention;
FIG. 5 is a schematic view of a lateral noise compensation system
in accordance with the present invention; and
FIG. 6 illustrates how changes in x and y can be identified by
collecting a single scan line from a triangle feature.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
FIG. 1 illustrates scanning probe microscopy (SPM) apparatus 1
including noise compensation apparatus. The apparatus 1 shown in
FIG. 1 includes a sample stage 2, adapted to receive a sample. A
probe 3 is mechanically connected to x, y, and z translation stages
5a for controlling fine movement of the probe 3 in all three
orthogonal directions. The translation stages preferably comprise a
set of piezoelectric transducers arranged to effect independent
movement of the probe in the three orthogonal directions. One or
more separate coarse positioning drives 5b are also provided for
driving the initial approach of the probe 3 to the sample on the
sample stage 2. The coarse positioning drives 5b optionally consist
of orthogonally arranged actuators. The coarse positioning drives
5b and the translation stages 5a are, in turn, in communication
with and controlled by a controller 6. The translation stages 5a
and positioning drives 5b provide fine and coarse positioning of
the probe 3 relative to the sample stage 2 respectively and can be
implemented in many different configurations and with many
different position devices. For example, although the translation
stages 5a and the positioning drives 5b are shown in FIG. 1
arranged to move the probe 3, it may be preferable to move the
sample stage 2 or split axes of the translation stages 5a and
positioning drives 5b between moving the sample stage 2 and probe
3. Moreover, it will be appreciated that the above and a piezo tube
scanner are only examples of the many different position control
systems that may be implemented with SPM apparatus.
As is conventional in the field, the z axis of a Cartesian
coordinate system is taken to be that perpendicular to a plane
occupied by the sample. That is, the probe--sample interaction is
dependent both on the x,y position of the probe 3 with respect to
the sample, and also on its height (z axis) above the sample.
The microscopy apparatus 1 includes a probe deflection detection
mechanism 7 which is arranged to monitor deflection of the
cantilever of the probe 3 for example employing an optical lever
technique in which a laser beam, reflected off the back of the
cantilever, is incident on a split photodiode comprising of two
segments. The point of reflection has a known or predictable
spatial relationship with respect to the tip of the probe. By
monitoring the difference in the intensity of light on the two
segments of the photodiode the position of the reflected light beam
and thus the deflection of the cantilever can be monitored. The
deflection detection mechanism 7 forms part of a feedback mechanism
whereby the controller 6 is supplied with the output from the
deflection detection mechanism 7 and on the basis of that output
the controller 6 controls the translation stages 5a to adjust the
separation of the probe from the sample surface so as to maintain a
predetermined deflection of the probe cantilever.
To generate an image or map of the interaction of the constant
deflection probe with the surface of the sample a height detection
system 9 is used to monitor the fluctuating height of the probe.
The position of the height detection system 9 is fixed in relation
to the support for the sample stage but the microscopy apparatus
described above is susceptible to any uncertainty in the spatial
relationship and thus the mechanical path between the sample stage
2 and probe 3 and the mechanical path is affected by environmental
noise particularly with respect to the vertical position of the
probe relative to the sample. To address this susceptibility the
height detection system 9 shown in FIG. 1 includes means for
compensating for environmental noise affecting the z position of
the probe. The height detection system 9 employs a differential
interferometric technique for monitoring the height of the probe 3
which has the advantage of providing self calibrated height data
with respect to the wavelength of the light source. Additionally,
if the reflected light has sufficient intensity the interferogram,
and thus the displacement measurement, is not affected by spurious
reflections from the surface of the sample. Thus the height
detection system 9 is preferably based on a two-beam
interferometric design which generates a measure of the absolute
height of the probe and which inherently incorporates compensation
for environmental noise.
With the two-beam interferometric design of the height detection
system 9 a first beam, the probe beam, is reflected off a point
which is in fixed spatial relationship to the probe tip, most
commonly the back of the probe. Simultaneously a second beam, the
reference beam, is reflected off a predetermined position which is
subject to the same or substantially the same externally applied
vibration as the sample for example on the support for the sample
stage, the sample stage or the sample itself. Ideally, the probe
beam and the reference beam are generated from a single common
light source. Whilst the surface of a sample is scanned by the
microscopy probe and the fluctuation in height of the probe is
monitored by the probe beam, the reference beam is not scanned
relative to the sample surface i.e. neither the x,y position of the
height detection system nor the x,y reference position on the
sample stage support, sample stage or sample, which is the point of
reflection for the reference beam, is changed. After reflection the
two beams are interferometrically combined to generate a
differential height measurement output from the height detection
system 9 which is directly representative of one or more
characteristics of the sample surface or, as in the illustrated
example where a feedback mechanism is used to maintain a constant
interaction, the differential height measurement is directly
representative of the topography of the sample surface or other
changes in a characteristic of the sample affecting the interaction
measurement.
In this example the support member for the probe has a low spring
constant parallel to the z direction. This results in the frequency
content of the height information output by the detection system
potentially including height information with frequencies greater
than the first resonant mode of the support member. Thus, by means
of a low spring constant probe cantilever in combination with a
differential interferometric device the range of variation of the
interaction force between the probe and sample is minimised and
also damage to the surface of the sample can be minimised. In this
way high resolution topographical data of the sample surface can be
collected. Thus the spring constant of the support member is chosen
to be below 2 Nm.sup.-1 and preferably below 1 Nm.sup.-1, more
preferably below 0.1 Nm.sup.-1. It will, of course, be understood
that alternative probes, for example those with much higher spring
constants, may alternatively be employed depending upon the nature
of the interaction to be studied, the characteristics of the sample
and/or the performance requirements for the microscopy
apparatus.
Additionally, as described in International patent application No.
WO2005/008679, the contents of which is incorporated herein by
reference, to aid the probe tip imaging the surface of the sample,
the mechanical properties of the probe may be altered, for example
by coating at least part of the probe in a mechanical energy
absorbing polymer and applying an external force directly to the
probe tip.
In this example, during a scan the z position or probe height might
not be altered by either the coarse translation drive 16 or any
other movement control, even where probe deflections in excess of
100 nm are measured, although it is preferable to minimise the
change in average interaction between the probe and sample. In
order to maintain an average interaction between the probe and the
specimen surface which would involve substantially constant probe
deflection, optionally, the position of the probe support member
may be adjusted during a scan.
With reference to FIG. 2, scanning probe microscopy apparatus in
the form of a surface review tool 10, specifically for use as a
semiconductor wafer review tool is shown schematically. The review
tool 10 generally comprises: a cantilever probe on a probe mount
11; one or more translation stages 12 for controlling movement of
the at least one probe in x, y and z orthogonal directions; and a
height detection system 13 for monitoring the fluctuating height of
the probe. A wafer support 14 is also illustrated but the relative
separation of the review tool 10 and the wafer support 14 is not to
scale, for ease of reference. The probe comprises a cantilever
support member and a nanometric probe tip which preferably has a
tip radius of 100 nm or less and with different sizes and shapes of
probe tip being employed for different types of scanning or image
collection.
The differential interferometer of the height detection system 13
is adapted to have a dynamic range of greater than the expected
range of height variations of the probe during a scan of the
sample. In practice, the height variations are often well in excess
of 1/2 wavelength of the probe and reference beams, especially in
the case of wafer review. Thus, the interferometer described herein
is adapted to have a dynamic range of greater than 1/2
wavelength.
The review tool 10 additionally includes an optical imaging system
15 for enabling a wafer to be imaged at visible/UV wavelengths; a
probe height control in the form of a z direction translation drive
16, such as a stepper motor, for the coarse control of the probe in
the z direction, in other words control of the separation or height
of the probe above the surface of the wafer; optionally a focussing
control 23 for controlling the focussing of the height detection
system 13 and/or the optical imaging system 15; a probe scanner 18
which controls the scanning movement of the probe relative to the
wafer; and a probe controller 17 which automatically controls, by
means of the translation stages 12, the alignment of the microscope
probe with the height detection system 13 and the wafer by means of
data input from the height detection system 13, the optical imaging
system 15 and optionally probe position feedback data.
The wafer support 14 preferably includes a movable stage 22 which
controls movement of the wafer, in x and y orthogonal directions
with an upper surface 20 on which the wafer 21 is mounted.
The basic principle of operation of the review tool 10 is as
follows: a wafer is positioned beneath the review tool by means of
the movable stage 22. The height detection system 13 is aligned
with the back of the probe cantilever and the probe is then lowered
towards the wafer using the z direction coarse translation drive 16
and fine adjustment of the probe position using the translation
stage 12. The probe is brought close enough to the surface of the
wafer for the support member or cantilever of the probe to deflect
as a result of an interaction with the surface of the wafer.
Once the desired level of interaction force, and hence deflection
of the probe cantilever, is established the probe is scanned over
the surface of the wafer by means of the probe scanner 18 whilst
the height of the probe is monitored by the system 13. In a scan
the probe is moved relative to the surface of the wafer in a line,
for example in either of the x or y directions, and measurements
are taken by the height detection system 13, along the scan line
either continuously or at intervals, of the probe cantilever
height. The probe may also be translated in a direction orthogonal
to the scan line i.e. the y or x directions, so that an area of the
surface of the specimen is scanned. Thus, surface area scanning is
performed involving continuously varying x and y probe positions.
Although reference has been made to separately scanning in two
orthogonal directions, more complex scanning movements of the probe
involving simultaneous x and y translations are also envisaged. The
measurements in terms of variations in the monitored height of the
probe cantilever are representative of one or more characteristics,
such as the topography, of the wafer surface and so can be used to
produce a 3D map of the scanned surface of the wafer. The probe
scanner 18 may also include a resonator 19 so that the probe can be
scanned across the surface of the wafer by means of oscillation of
the probe at or near its resonant frequency in the scanning plane.
By oscillating the probe at or near its resonant frequency the
oscillation amplitude accurately and repeatably determines the
length of the scan line. In combination with monitoring the
fluctuations in height of the probe during the scan, the height
detection system 13 monitors vibration of the sample in the z
direction and extracts differential probe height data. The
differential height data output by the review tool 10 is
representative of the absolute or true height of the probe relative
to the sample and inherently includes compensation for noise in the
form of vertical vibrations arising from external environmental
factors, for example from the wafer fabrication production line.
The wafer review tool 10 may additionally include conventional
means (not shown) for ensuring alignment of the height detection
system 13 with the probe and also signal synchronisation means for
ensuring synchronisation of the differential height data generated
by the height detection system 13 with position data identifying
the lateral (x,y) positions of the probe during collection of the
differential height data.
Although reference is made herein to an optical imaging system 15
in addition to the height detection system 13, it is envisaged that
with the review tool, optionally, only a part of the optical
imaging system 15 may be used enabling the review tool 10 to be
integrated with or retro-fitted to existing optical microscopy
apparatus in the wafer production line. Integration of the optical
imaging system and the height detection system offers the advantage
that optical images of the wafer surface and images produced using
the probe can be directly and automatically aligned and
compared.
In FIGS. 3 and 4 two different designs for noise compensation in
the z direction are illustrated employing a feedback control 26 for
maintaining constant probe/sample interaction. These designs are
suited for use with local probe apparatus utilising low spring
constant cantilevers as described earlier amongst other probe
microscopy arrangements. In FIG. 3 light, preferably from a common
coherent light source (not shown), is split so that a first beam is
reflected off the back of the probe cantilever of the local probe
apparatus and a second beam is reflected from a fixed x, y position
on the specimen support, for example, with a reflector 22a. The use
of a separate fixed reflective element is useful where the surface
of the specimen is non-reflective or is so irregular as to fail to
provide a return beam. The two reflected beams are then combined in
an interferometer 9 to provide a differential measurement of the
variations in the height of the probe cantilever of the local probe
apparatus relative to the reflector 22a. As the fixed x, y position
on the reflector 22a is subject to the same vertical vibrations as
the x, y positions on the sample surface scanned by the local probe
apparatus, these vertical vibrations are cancelled out in the
differential interferometric measurements of the probe height with
respect to x, y position as the probe is scanned across the sample
surface.
In FIG. 4 again differential interferometric measurements of the
probe height are recorded. However, with this design the reference
height is recorded with respect to a reference probe 24 aligned
with a fixed point on the sample. The reference probe 24 is similar
to the measurement probe 3 and ideally matches the measurement
probe in terms of performance characteristics. To ensure that the
reference probe 24 is exposed to substantially the same mechanical
vibrations as the measurement probe 3, the reference probe 24 is
supported by separate translation stages 25 that are kept inactive
during a scan of the surface of the sample so as to fix the
position of the reference probe 24.
The noise compensation designs illustrated in FIGS. 3 and 4 are
particularly suited for use with the wafer review tool of FIG. 2 as
the interferometer of the review tool may be employed to
additionally correct for vertical noise at around sub to a few
hundred Hertz arising from external environmental vibrations which
are prevalent in a wafer fabrication production line. Furthermore,
the use of differential interferometric measurements in noise
compensation, as illustrated in FIGS. 3 and 4, is also particularly
suited to rapid probe engagement with the surface which is a key
aspect of any wafer review tool on a wafer fabrication production
line. In this configuration the separation between the probe and
sample is directly measured which enables the probe to approach the
sample at high speed without the risk of contact.
The noise compensation designs described above are specific to the
correction of noise in the z axis of the local probe apparatus.
However, the same or similar techniques may also be employed for
correcting for lateral noise i.e. environmental vibrations in the x
and y axes. As shown in FIG. 5, to correct for lateral noise, a
first position sensitive detector 50, such as an optical
displacement sensor, is arranged to monitor for changes in the
separation, substantially parallel to the x axis, of the position
sensitive detector 50 from a first fixed position which is subject
to the same or substantially the same lateral vibrations as the
sample e.g. on the side of the sample stage, the sample stage
support or a wafer in the case of a wafer review tool. Thus, change
in the separation between a pair of predetermined fixed positions
is monitored. Similarly, a second position sensitive detector 51 is
arranged to monitor for changes in the separation, substantially
parallel to the y axis, of the position sensitive detector 51 from
a second fixed position which again is subject to substantially the
same vibrations as the sample such as on the side of the sample
stage, the sample stage support or the wafer. The position of the
first and second fixed points are monitored respectively by the
first and second position sensitive detectors 50, 51 to provide
reference positions P.sub.x and P.sub.y which are synchronised with
the probe height measurements generated by the scanning probe
microscopy apparatus.
Where any change in P.sub.x and/or P.sub.y is detected, the value
of this change is used as a noise compensation in the x,y position
of the synchronised measured probe height. The noise compensation
may be performed in one of two ways: (i) the change to P, and/or
P.sub.y may be applied as an offset correction to the trajectory of
the scan of the probe in which case no post processing of the probe
height measurements is required; or (ii) the placement of the probe
height position measurement(s) as one or more data points in a
pixellated array representative of the surface of the sample or
wafer may be offset by the amount of the change in P.sub.x and/or
P.sub.y. In the latter case, probe height measurements are usually
continuously recorded during scanning by the local probe apparatus
of the sample surface. As a result, a plurality of probe height
measurements are recorded for each pixel of the array
(over-sampling) and are then integrated within a pixel and across
pixels which enables the offsetting of the data point(s) to be
accurately represented in the final image of the surface of the
sample or wafer generated using the differential probe height
measurements. Reference is made herein to the use of optical
displacement detection in correcting for noise arising from lateral
environmental vibrations, this could be in the form of
interferometric or optical encoder measurements of the P.sub.x and
P.sub.y positions of the sample stage or the wafer. Any
displacement measurement technique with the necessary resolution
and bandwidth can be used provided the dynamic displacement range
of the measurement is greater than the amplitude of the vibration
to be corrected.
In the case of a wafer review tool, the x, y positioning
information used to control the initial positioning of the wafer
stage beneath the review tool may also be employed for the noise
compensation system. Moreover, in a high throughput environment,
the use of lateral position sensors 50, 51 enables the sample or
wafer to be imaged whilst the sample/wafer stage is still settling
because the outputs of the lateral position sensors 50, 51 enables
any variation in scan position arising from settling of the
sample/wafer stage to be detected and accounted for in the image of
the sample/wafer surface.
Instead of a conventional optical displacement sensor a
differential interferometer may be used as an alternative method to
monitor lateral vibrations. A first beam of the interferometer is
reflected from the sample, the sample stage or the sample stage
support and a second beam is reflected off a position in a fixed
spatial relationship to the probe such as the probe support. The
two beams are them interferometrically combined to generate a
differential signal representative of vibration in a plane parallel
to the imaging plane of the microscope.
A further alternative method of measuring changes in the lateral
sample position relative to the measurement scanning probe is to
use a scanning lateral reference probe imaging a reference
structure which is subject to the same or substantially the same
lateral vibration as the sample. The scanning lateral reference
probe has a fixed lateral position relative to the measurement
probe so that any change in the lateral position of the sample will
affect both probes equivalently. The scanning lateral reference
probe images a predefined structure from which any change in
lateral position can be determined i.e. any change in the position
of the reference structure parallel to the scanning plane. The
maximum vibration frequency component that can be corrected is
directly related to the rate at which the lateral position can be
monitored and so it is necessary that the scan rate of the scanning
lateral reference probe be greater than the measurement probe and
preferably greater than the desired pixel collection rate. By
appropriate choice of reference structure it is possible to negate
the need to scan the reference in both x and y. For example, FIG. 6
demonstrates that both changes in x and y can be obtained by
collecting a single scan line from a triangle feature. The
displacement in point A and width (W1-W2) provides a measurement of
displacement of the sample in the x and y axes respectively.
Thus, it may be seen that with the present invention, greater image
resolution of a local probe microscope can be achieved through the
cancellation of external environmental vibrations and that this is
of particular importance where local probe microscopy apparatus is
employed in a wafer review tool of a wafer fabrication production
line. Moreover, as the differential interferometric measurements of
the wafer review tool provide true-height information, 3D
information on wafer feature and defects can be obtained.
Furthermore, with the present invention local probe surveying and
imaging techniques can be combined with conventional semiconductor
wafer inspection and review techniques and the review tool may be
integrated with existing inspection and review apparatus which
otherwise may not be possible or at a reduced level of resolution
and performance as a result of the noise generated by the existing
inspection and review apparatus.
Although reference has been made specifically herein to the present
invention being employed in wafer inspection and review, it will be
apparent that the present invention is equally applicable to the
use of local probe microscopy in a noisy environment and also
wherever information on surface topography, such as true-height
information, is required as well as other surface characteristics.
For example, the present invention may be used as a survey and/or
imaging tool in measuring step heights, surface roughness, film
thicknesses, critical dimensions and other metrology
applications.
Furthermore, the interaction between the probe tip and the surface
of the specimen may involve any spatially varying measurable
property including one or more of the following: interatomic forces
both attractive and repulsive, magnetic, electric, electromagnetic,
electron tunnelling, capacitative, thermal interactions, lateral
forces or shear forces. The probe sample interaction may also be
monitored in quasi-static or dynamic modes of operation, for
example, contact, intermittent contact and non-contact.
It should also be borne in mind that although the review tool
described above includes a local probe imaging system and an
optical imaging system, it is not essential for image data, as
such, to be generated by these systems. Instead, for example, the
local probe imaging system may only generate probe height
information with respect to probe position that is used, for
example, in comparative analysis. Also, as mentioned earlier, the
probe height information may not be representative of the
topography of the specimen surface but may, instead, be
representative of one or more other characteristics of the specimen
surface. Also, in the case of the optical imaging system, here too
it is not essential for the system to generate image data. For
example, the optical imaging system may be used for the purposes of
alignment of the review tool with the specimen surface.
When measuring different interactions under different measurement
conditions, e.g. in a dynamic mode of operation, rather than
reflecting the probe beam off the end of the probe cantilever (as
illustrated), the probe beam may be reflected off alternative parts
of the apparatus. Additionally, although the reference beam is
shown being reflected off an upper surface of the sample or sample
support, it is envisaged that the reference beam may be reflected
off other positions which experience substantially the same
vibration e.g. the rear of the sample.
Although the use of piezoelectric actuators for control of the
movement of the imaging system and the reference height sensor
probe is preferred, other actuators involving for example thermal
expansion of a control rod, or a voice coil are envisaged.
Similarly, alternative components and arrangements to those
described herein may be employed without departing from the scope
of the invention as defined in the accompanying claims.
* * * * *